This invention relates generally to ink jet printing and specifically to an ink jet printing apparatus and method for printing alpha-numeric characters onto a moving surface, such as containers moving along a conveyor.
Ink jet printing devices can be broadly divided into two categories, continuous and so called "on-demand" technology. In continuous ink jet printing devices, an ink manifold houses a piezoelectric crystal which vibrates upon application of an electric signal. The vibrations force ink out a nozzle past an electrode which charges selected drops. The ink stream continues through charged plates where the charged particles are deflected upward onto a target. The unchanged drops fall into a collection tray and are recirculated. Such devices are generally used for high speed printing with quality comparable to a typewriter and all rely upon a continuous, synchronized stream of ink drops.
On-demand ink jet printing devices utilize some variation of a technique in which ink drops are issued only on demand rather than in a continuous stream. Typically, there is provided a plurality of nozzles through which ink can be intermittently selectively ejected. Prior devices of this kind have generally comprised either piezoelectric ejector means associated with each nozzle for ejecting ink therethrough or an electrically operated needle valve or plunger for opening and closing each nozzle. In either case, the apparatus is relatively complex and uses only parts which have to be specially manufactured with the result that the apparatus tends to be extremely costly. Such devices were not well suited for the packaging industry where larger characters, for instance, characters having a height of from 13 mm to 70 mm are printed on containers moving down a conveyor line. There has existed a need, therefore, for a device which is simple and robust in construction, and which can be manufactured using commercially available parts to withstand the rigors of a packaging or assembly line environment.
In addition, a problem has existed in on-demand devices in providing a suitable ink which will not dry in the ink jet nozzles during their intermittent operation, yet which has good wetting properties on the printing substrate and a suitable viscosity. Certain of the designs previously suggested, although simpler in construction, did not prove satisfactory in operation because of the failure to match ink with apparatus.